1. Field of the Invention
This invention relates generally to spreading the clamping load of a bolt which holds an inner liner to an outer shell used in ore milling operations, sealing the shell so that the wet ore slurry in the inner lining does not leak out of the shell through the mounting holes, and repairing enlarged mounting holes which go through the liner and shell to re-establish the ability of the bolt to maintain its designed and desired preload.
2. Description of the Related Art
a. Prior Mill Liner Bolting Techniques and Problems
Because of individual geometry, the head shape of a conventional tapered or spherical head, counter sunk mill liner bolt, presents a poor load bearing contact with the liner and its mating surface. This condition is due primarily to clearance and manufacturing tolerances between the liner, which is cast, and the radiused, tapered, load bearing area of the bolt, which is forged. This poor load area contact always causes early embedment with subsequent loss of bolt clamp load soon after assembly. It requires repeated, expensive mill shut downs, for bolt tightening, before an adequate surface contact can be formed to support the required bolt clamp load. In a majority of instances, bolts never establish enough surface area contact to stay tight and have to be periodically tightened through the life of the liners. Often, loose bolts fatigue and break with expensive consequences if the liners fall out.
While it is known to provide round headed bolts with a plurality of hard metal protrusions of various types, i.e., wings, keys, or fins, formed along the neck and shank of the bolt which embed themselves in the softer work piece to prevent the bolt from turning when a load is applied to the bolt, such structures are unnecessary when a non-round bolt head is supported in a non-round recess and, moreover, will not work in the environment of the present invention where the work piece, i.e., the mill liner, is made of harder steel than the bolt of the fastener.
Accordingly, it is an object of the present invention to provide improved bolt tightening features and processes which provide improved mating surfaces and preload holding ability which overcomes the problems of the prior art.
Moreover, running with loose bolts is also the cause of bolt holes in the mill shell becoming enlarged, which encourages further loosening and leaking problems eventually leading to expensive repair. Typically, the bolt holes in the liners are cast approximately 1/8" larger, all around the bolt head, for a standard 11/2" liner bolt. FIG. 3 shows how inadequately the bolt load surface mates with the liner in a horizontal (radial) plane. FIGS. 1, 2 show loss of vertical (axial) contact face with small neck angle differences.
Accordingly, it is an object of the present invention to provide improved bolt tightening mechanisms which prevent loosening and enlargement of the holes through which the bolt passes thereby overcoming the problems of the prior art.
b. Prior Liner Bolt Sealing Problem
Prior sealing systems used with liner bolts are designed to give good sealing characteristics, but in doing so, sacrifice jointing efficiency. Typically rubber is introduced between the joint surfaces so that a hard joint, i.e., metal-to-metal, is never attained. This condition then becomes a gasketed joint and will never hold bolt preloads adequately.
While it is known to provide a repair washer with a metal-to-metal contact using a two- or three-piece metal and rubber washer combination atop a repair washer to effect an outboard seal at the outer circumference of the bolt of the fastener and the outboard end of the central bore of the repair washer, such structure ignores the further need for a simultaneous metal-to-metal contact with a resilient member beneath the repair washer to effect an inboard seal at the inboard end of the central bore of the repair washer.
Accordingly, it is an object of the present invention to provide an improved sealing system without sacrificing joint efficiency by providing metal-to-metal contact with simultaneous adequate sealing thereby overcoming the problems of the prior art.